The present invention relates to an industrial purification process in the process of manufacturing alcohols and ketones by oxidation of hydrocarbon wherein after removing by extraction organic impurities, formed as by-products of an oxidation reaction, from aqueous boric acid obtained by hydrolysis of an oxidation reaction solution containing boric ester, obtained by performing a hydrocarbon oxidation reaction in the presence of boric acid with gas containing molecular oxygen, using an organic polar solvent having an oxygen atom within its molecule, boric acid is regenerated and cyclically used again in said hydrocarbon oxidation reaction. As examples of usage of alcohols and ketones, in the case of cyclododecanol and cyclododecanone, they are used for the intermediate starting material of laurolactam, a monomer used in the manufacturing of 12-Nylon and for the starting material of a dodecane dibasic acid, and in the case of cyclohexanol and cyclohexanone, they are used for the intermediate starting material of caprolactam, a monomer used in the manufacturing of 6-Nylon and for the starting material of adipic acid.
Various methods of the prior art are known for purifying boric acid used in a process wherein boric acid that has been regenerated from aqueous boric acid, obtained by hydrolysis of an oxidation reaction solution obtained in a hydrocarbon oxidation reaction performed in the presence of boric acid, is used cyclically. Examples of known methods for purifying aqueous boric acid obtained by hydrolysis include steam stripping the aqueous boric acid obtained by hydrolysis of an oxidation reaction solution to remove organic impurities contained therein (see U.S. Pat. No. 3,423,571). However, the above-mentioned known steam stripping method has the disadvantage of being unable to remove a majority of the organic impurities, which have a detrimental effect on the oxidation reaction, even if a considerably large amount of steam is used.
In addition, examples of known methods wherein aqueous boric acid obtained by hydrolysis is purified with solvent include a method wherein aqueous boric acid obtained by hydrolysis is removed by extraction with saturated, linear or cyclic hydrocarbons such as cyclododecane, cyclohexane, cyclopentane, n-decane, n-heptane and n-hexane (see U.S. Pat. No. 3,679,751). However, as nearly all of the organic impurities contained in said aqueous boric acid are polar compounds, methods which use nonpolar solvents like the above-mentioned hydrocarbons for the extracting solvent have the disadvantage of being unable to achieve a satisfactory removal rate during removal of those impurities by extraction.
Moreover, examples of known methods wherein aqueous boric acid obtained by hydrolysis is crystallized include a method wherein boric acid crystals are removed from aqueous boric acid obtained by hydrolysis by crystallization, and organic impurities are removed by wet oxidation of the mother liquor formed as a result of said crystallization (see Japanese Patent Publication No. 4524/1977). This proposed method has the disadvantage of requiring high temperature and high pressure apparatus resulting in increased apparatus complexity and high equipment costs. In addition, another example of a known method wherein aqueous boric acid obtained by hydrolysis is crystallized is a crystallization purification method wherein only crystals obtained by removing boric acid crystals in the first stage of crystallization are cyclically used for an oxidation reaction as a result of two-stage crystallization from aqueous boric acid obtained by hydrolysis (see Japanese Patent Publication No. 39250/1970 and Japanese Unexamined Patent Publication No. 29299/1978). However, accumulation of organic substances is prevented by redissolving and returning the crystals obtained in the second stage of crystallization either to the first stage crystallization process or the hydrolysis process, and discarding a portion of the mother liquor of second stage crystallization while a portion of the second stage crystallization outside the system. Thus, this method has the disadvantage of being undesirable in terms of costs and the environment as a portion of the aqueous boric acid must be discarded. Thus, all of the methods of the prior art were not able to be satisfactory in terms of industrial use.